1. Field
The invention is related to a fiber optic cable, and more particularly to a high temperature zero fiber strain fiber optic cable that can be deployed in oil and gas well applications.
2. Related Art and Background
There is a need for high temperature, zero fiber strain, fiber optic cables that can be deployed in high temperature environments, such as oil and gas wells. These cables may be deployed directly in the wells as opposed to in a coiled tube that is then deployed in the well.
Some conventional cables use stranded tubes where the tubes are made with ETFE (Teflon). These designs have been problematic in the field for several of reasons. One is that Teflon and other plastic compounds are known to evolve hydrogen at elevated temperatures. Hydrogen evolution is detrimental to the optical performance of the cable. Another issue is the upper temperature is still limited to a maximum of 150 degrees C. Though there are some polymers that are capable of going to higher temps, such as 300 degrees C., none can go to 600 C or even higher.
When fiber optics are deployed via coiled tubing they must first be installed into the coiled tube. There are various methods defined in prior art for accomplishing this installation however all of them require equipment and processes that add complexity, risk, and cost. Having a fiber optic element that is able to be directly deployed via injection negates the need for these added processes.
In order to be deployed directly in a well, the cable structure must have an adequate cross sectional area and modulus to provide sufficient buckling strength to allow for injecting under force (pushing) into a deviated well bore. This type of deployment is not commonly achieved with existing fiber optic cable designs. However, the invention, shown in the embodiment in FIG. 1—utilizing 1 or 2 tubes and 3 or 4 solid metallic fillers provides for adequate strength to be directly injected into a wellbore. Another type of directly deployable cable in a vertical wellbore is known as a “slickline”. A slickline cable has a solid circular outer surface which allows the cable to be sealed off at the well head using a packer. It is much easier and less costly to manage wellbore pressure using this type of cable and seal. Slickline cables are typically lowered into a vertical wellbore by their own suspended weight. Commonly, a weight bar is added to assist in the initial descent. A common problem with slickline cables is the metallic tubing will fatigue and fracture after a certain number of cycles. If the tubing fractures in the well this leads to significant challenges “fishing” the remnant out of the wellbore. A typical solution to enhance the fatigue life for slickline cables is by using duplex alloys or other high endurance alloys that can be conditioned by heat treatment. Heat treatment of the metal tubing is not an option when the cable contains a fiber optic or copper core as the temperature of the heat treatment exceeds the temperature rating of the core.
Therefore there is a need for a cable that has the following features. Although the invention is not limited to a cable that has these features.                1. High temperature capability—zero fiber strain up to at least 600 degrees C.        2. Exceptionally high crush resistance useful for permanently installed cables in open hole.        3. High buckling strength for injection into horizontal or high pressure wells.        4. Enhanced sensitivity to thermal and acoustic energy through proximity of fiber tube to outer surface.        5. Mechanically robust to improve resistance to hydraulic fracturing fluids.        6. Construction allows for use a high strength slickline logging cable.        7. Higher fatigue resistance for longer service life in dynamic applications.        